File systems store files and store information about files. The information stored in files may be referred to as data. The information about files may be referred to as metadata. The metadata may include, for example, a file name, a file size, a file parent, a file descendant, a file access time, a file owner, file permissions, and other information. Some of the metadata for an individual file may be stored in a data structure known as an inode. The inodes and metadata for a file system may also be stored collectively. The metadata has both structure and content. When the data in a file or about a file changes, a file system may want to update the metadata about that file. For example, if the contents of a file are changed, the file system may want to memorialize the time at which the change was made and by whom the change was made. Actions on a file produce actions on a corresponding inode.
Inodes are local to a file system. They are a private resource of a file system. Thus, actions performed on a file in a file system produce actions on a corresponding inode associated with that individual file system. However, individual file systems acting in isolation are becoming more rare. Or, seen from another point of view, collections of file systems acting together are becoming more common. For example, in geo-spreading, files are spread across a system but stored in separate local file systems, where each file system has its own inodes. Although each file system has its own inodes, it may be desirable for a first file system to understand actions that were taken on files or inodes in a second file system.